1. Field of Invention
The invention relates to a pneumatic cartridge expressing device for expressing cartridges containing viscous materials, said cartridges consisting of a tubular body, said body being open at the rear and sealed at the front by an end wall with a mouthpiece, and a piston, said piston being inserted into the rear of the tubular body and being axially displaceable therein.
One particular area of application of the invention is expressing printing ink cartridges for filling ink fountains on printing presses. However, it is equally possible to conceive of an application for example that entails expressing cartridges filled with sealing materials, adhesives, etc. at fixed workstations or by means of manually operated devices.
2. Description of Related Art
The pneumatic expressing of printing ink cartridges in the ink fountains of printing presses is already known in practice. This is usually performed indirectly by a pneumatic cylinder charged with compressed air, the piston of said cylinder actuating a mechanical plunger that abuts at its front the piston of the cartridge to be expressed, advancing said piston. The disadvantage of these pneumatic cylinder devices consists in their cumbersome, heavy, and expensive mechanisms.
However, attempts have also been made to perform pneumatic expression of cartridges by having the compressed air act directly on the cartridge piston. This requires an air-tight seal at the rear of the tubular body of the cartridge. For this purpose, one known proposal provides a tubular housing into which the cartridge is inserted with the mouthpiece facing downward and the rear of the tubular body of the cartridge facing upward. A cap with a compressed air connection is then placed on the open upper end of the tubular housing, said cap being locked by a bayonet-lock-type mechanism to the housing. The cap has a sealing edge that abuts the rear of the tubular body of the cartridge in order to prevent compressed air from escaping from the chamber that is located behind the cartridge piston and communicates with the compressed air connection of the cap.
However, the seal at the rear end of the cartridge poses problems. Since the tubular bodies of the cartridges are usually made of plastic and have a considerable length of about 30 cm or more in the case of ink cartridges, because of the material properties of the plastic that is used, especially its shrinking behavior, said bodies exhibit considerable lengthwise tolerances. As a result, it may happen that when the cap is tightly fitted, the sealing edge does not fit tightly against the edge of the rear tubular body of the cartridge, or in the case of a cartridge with a relatively large positive lengthwise tolerance, the cap cannot be fitted properly. Another serious problem consists in the fact that the compressed air acting on the piston attempts to bulge the tubular wall of the cartridge, which results in leaks between the piston and the tubular wall of the cartridge, especially if the piston is located approximately in the middle of the length of the cartridge.